A charged particle microscope has a very high resolution as compared with an optical microscope, and is widely used in order to clearly observe a fine structure of an object to be observed. In a charged particle microscope, a target sample is irradiated with a charged particle beam, and a particle (a charged particle of the same type as or a different type from the emitted charged particle applied to the target sample, or electromagnetic wave, a photon) that is emitted from the target sample or transmits through the target sample are detected by a detector, thereby capturing an enlarged image of the target sample.
In particular, in a semiconductor manufacturing process, charged particle microscopes, such as a scanning electron microscope, a scanning ion microscope, a scanning transmission electron microscope, are used as the applications, such as inspection of a semiconductor wafer, measurement of a pattern size, and measurement of a pattern shape. In these applications, the observation of a semiconductor pattern or a defect, the detection of a defect, the cause analysis of a defect, the measurement of a pattern size, and the like are performed using a captured image.
In the charged particle microscope, a function to provide a high quality image is one of the most important functions. On the other hand, in recent years, as the sizes of semiconductor patterns further decrease and the density of semiconductor patterns further increases, it is becoming difficult to detect a sufficient amount of particles emitted from underlayer regions (regions other than upper layer patterns) in a target sample with a multilayer layer and thus there is a problem that visibility significantly drops, such as that the S/N and/or contrast in an underlayer region decrease. Similarly, also in a hole pattern, it is difficult to detect particles from the bottom of a hole and therefore an excellent image quality cannot be obtained in the bottom region of a hole. A region, such as an underlayer region or a hole pattern, where a sufficient signal amount (a detected amount of particles emitted from a sample) cannot be obtained, is referred to as a dark part. It is also possible to detect more particles emitted from a dark part by improving a charged particle beam, with which a sample is irradiated, and/or an electromagnetic field distribution in a vicinity of a sample, but there is a limit in increasing a signal amount to be obtained. Moreover, there is also a method for achieving an increase in image quality by image processing a captured image (e.g., Patent Literature 1). Moreover, in an optical camera, an optical microscope, and/or the like, there is proposed a method for imaging with a varied exposure amount in order to clearly display a dark part (e.g., Patent Literature 2). An increase of the exposure amount in an optical microscope or an optical camera corresponds to an increase of the irradiation amount (hereinafter, dose amount) of a charged particle beam in a charged particle microscope.